c++: CWG 2789 and usings [PR116492]

c++: CWG 2789 and usings [PR116492]

After CWG 2789, the "more constrained" tiebreaker for non-template
functions should exclude member functions that are defined in
different classes.  This patch implements this missing refinement.

In turn we can get rid of four-parameter version of object_parms_correspond
and call the main overload directly since now correspondence is only
only checked for members from the same class.

	PR c++/116492
	DR 2789

gcc/cp/ChangeLog:

	* call.cc (object_parms_correspond): Remove.
	(cand_parms_match): Return false for member functions that come
	from different classes.  Adjust call to object_parms_correspond.
	(joust): Update comment for the non-template "more constrained"
	case.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-memfun4.C: Also compile in C++20 mode.
	Expect ambiguity when candidates come from different classes.
	* g++.dg/cpp2a/concepts-inherit-ctor12.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>